Various methods have been employed to obtain high burning rate propellants. One method of obtaining very high burning rates from propellants has been through the use of aluminum staples. Other metals have been used with comparable results; however, aluminum staple of various dimensions has been preferred because of being readily available in both quality and quantity at a lesser cost than other metals.
In addition to staple, catalysts have been used to promote burning rates. At first, liquid burning rate catalysts (particularly of the organoiron or carborane types) were employed and these catalysts also, in certain cases, contributed plasticizer functions in addition to the catalysis function. Some problems associated with the use of such catalysts have been related to their relatively high volatility, their high freezing points, and their tendencies to migrate within the propellant and into the liner and insulation of the rocket motor. In order to overcome these migratory tendencies, it has been necessary to incorporate high percentages of burning rate promotor into the insulation to produce a near-equilibrium situation insofar as catalyst migration is concerned, thus further complicating the manufacture of these types of solid rocket motors.
More recently, my U.S. Pat. No. 4,034,675 disclosed another method of controlling burning rate while at the same time employing a means to eliminate combustion instability. Burning rate promotor (e.g. n-butylferrocene, n-hexylcarborane) is incorporated into resonance rods that are used to eliminate combustion instability of solid propellant. As the solid propellant burning progresses the resonance rods undergo ablation, and as ablation takes place, subsurface quantities of the burning rate promotor are exposed and continuously released into the combustion zone to catalyze the combustion process. An alternate embodiment employed porous resonance rods which contain the burning rate promotor and a selected material for suppressing combustion instability as infiltrants.
Burning rates of propellants are also influenced by surface area and particle sizes of the oxidizer ingredients. Porosity is another factor which increases burning rate of solid propellant grains. However, if porosity is not controlled the burning rate increase due to porosity as a result of processing abnormalities can lead to failure of the solid propellant grain.
Desirable would be a method to enhance the burning rate of a propellant formulation which has proven properties, both mechanical and chemical, without changing the stoichiometric balance of the formulation.
Therefore an object of this invention is to provide a mechanical enhancement of the burning rate of solid propellants by means of shrink tubes.
Another object of this invention is to provide a method of increasing the burning rate as a result of increased porosity generated, either before the propellant is ignited, or at the early burning stage of the propellant, as a result of thermally-shrinkable tubing which are incorporated into the propellant formulation.
Still another object of this invention is to provide thermally shrinkable plastic tubing which has been prior filled with a rapidly-combustible propellant prior to being incorporated into the matrix propellant which would burn more rapidly than the matrix propellant, and thus, produce even higher burning rates as a result of rapidly generated annular passageways in the propellant grain as a result of tubing shrinkage.